The present invention relates to a method and apparatus for monitoring vertical surface variations of specimens on a microscopic scale and more particularly to such a method and apparatus wherein the monitoring function is carried out in a non-destructive manner with respect to the specimen.
There are many situations where it is essential to accurately measure or assess vertical dimensions or variations in surfaces of specimens on a microscopic scale. For example, such techniques are of importance in surface treatment of recording elements, thin film measurements required in connection with various semiconductor devices and the like. In such devices, it is important to be able to obtain accurate measurements of vertical dimensions in the surface in a non-destructive manner. Otherwise, the simple measurement step itself could produce unacceptable surface blemishes on the specimen or could interfere with operating integrity of the specimen.
Other uses will also be immediately apparent for the method and apparatus of the present invention. For example, in many applications, it is necessary to determine whether scratches or other surface blemishes are of sufficient depth to interfere with proper operation or functioning of the specimen or device. In other applications, it is necessary to accurately measure or monitor the depth of features such as scratches which may be on the order of less than 50 microinches in width and one to ten microinches deep. One of the devices most commonly employed in the past for such measurements included a diamond stylus moved across the surface of the specimen with transducer means indicating variations encountered by the diamond stylus. However, because of the hardness of the diamond stylus, it tended to penetrate portions of the surface of the specimen thereby preventing accurate measurement while also leaving undesirable marks upon the specimen surface. In any event, such surface profile measuring microscope do not have the resolution necessary for measuring features of the type described above. Similarly, multiple beam interferometry devices have been employed for such measurements but, at a typical magnification of one hundred to two hundred times (100-200X), they are not capable of sufficient resolution for accomplishing such measurements.
More recently, other optical or non-destructive techniques have been employed for measuring or monitoring vertical displacement or dimensions on a microscopic scale. For example, reference may be had to an article entitled "A Topographic Microscope" by Samuel Tolansky appearing at pages 54-59 in the August 1954 issue of Scientific American magazine. The device disclosed in that article relied on the use of an offset or oblique source of illumination for casting a shadow line across the entire surface of a specimen to be analyzed. Variations in vertical displacement could then be detected or assessed from observing non-linearities in the shadow line itself. However, even though relatively high resolution is possible with such techniques, it is necessary in each instance to calculate vertical displacement from the particular non-linearity of the shadow line observed for selective surface features as well as the angular relation of the light or illumination source to the surface of the specimen.
Other prior art devices have been disclosed for accomplishing similar measurement techniques but they have been based on generally similar operating parameters as those described above. Accordingly, there has been found to remain a need for a non-destructive method and apparatus for measuring or assessing vertical displacement or dimensions on a microscopic scale in various specimen surfaces or devices.